Chrominance signal noise reduction systems may be categorized as belonging to one of two types, namely, those that operate on a quadrature amplitude modulated chrominance signal subcarrier directly and those that operate on demodulated color difference signals.
An example of noise reduction by processing modulated chrominance signal is described by Hirota in U.S. Pat. No. 4,558,353 entitled CIRCUIT FOR REDUCING NOISE IN A CARRIER CHROMINANCE SIGNAL which issued Dec. 10, 1985. The Hirota system includes circuitry for detecting noise in a narrow band region of a chrominance signal carrier wave and for subtracting the narrow band noise from the full bandwidth chrominance signal to provide a noise reduced chrominance signal carrier wave. It is stated that the noise component can be effectively eliminated especially when there is no change or only a small change in color, that is, when side band components of the carrier chrominance signal are small.
Another example of a chrominance noise reduction system which operates on the modulated chrominance signal carrier wave is described by Takahashi in U.S. Pat. No. 4,246,610 entitled NOISE REDUCTION SYSTEM FOR COLOR TELEVISION SIGNAL In this system frame recursive filtering is applied to a composite video signal to provide noise reduction in the temporal dimension (i.e., from frame to frame). To accommodate the presence of the color subcarrier in the composite signal being filtered, the system includes a chrominance signal inverter that reverses the chrominance phase from frame to frame. Additional processing circuitry is also employed to reduce chrominance signal artifacts produced as a result of frame to frame motion.
Baseband chrominance signal processing techniques are known wherein the chrominance signal is demodulated to baseband component form (e.g., R-Y and B-Y) prior to application of noise reduction. An example of base-band chrominance signal noise reduction is described by Kisou in U.S. Pat. No. 4,928,165 entitled NOISE REDUCTION CIRCUIT FOR A CHROMINANCE SIGNAL OF A USING FRAME CORRELATION (Sic.) which issued May 22, 1990.
The Kisou system includes a decoder for decoding first and second color difference signals, each including a noise component form a received chrominance signal. A signal subtraction circuit generates a first frame difference signal in response to two successive first color difference signals and a second frame difference signal in response to two successive second color difference signals. A signal converting circuit extracts a first compensation signal from the first frame difference signal and a second compensation signal from the second frame difference signal. A noise reduction circuit reduces noises from the color difference signals in response to the compensation signals. A discrimination circuit discriminates whether both the first and second frame difference signals are smaller than a prescribed level or not and generates a control signal in response thereto. A signal transmission circuit transfers the compensation signals from the signal converting circuit to the noise reduction circuit in response to the control signal only when both the frame difference signals are smaller than the prescribed level.
One problem with the Kisou arrangement is that since two chrominance signals are being processed a relatively large number of processing elements is required. In other words, there is a substantial duplication of elements. Also, the use of two frame memories for chrominance signal processing is relatively costly and complex.